# 054-spiral-matrix

## Question

https://leetcode.com/problems/spiral-matrix/description/

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

Example:

Given the following matrix:

``````[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]
``````

You should return`[1,2,3,6,9,8,7,4,5]`.

## Thought Process

1. Collect in the order top, right, bottom, left
2. We go from outer layer inward
3. Time complexity O(n)
4. Space complexity O(n), O(1) extra space

## Solution

``````class Solution {
public List<Integer> spiralOrder(int[][] matrix) {
List<Integer> res = new ArrayList<>();
if (matrix == null || matrix.length == 0) return res;
int m = matrix.length, n = matrix[0].length;
int top = 0, bottom = m - 1;
int left = 0, right = n - 1;
while (top <= bottom && left <= right) {
// collect the top row
for (int c = left; c <= right; c++) res.add(matrix[top][c]);
top++;
// collect the right column
for (int r = top; r <= bottom; r++) res.add(matrix[r][right]);
right--;
// collect the bottom row
for (int c = right; c >= left; c--) res.add(matrix[bottom][c]);
bottom--;
// collect the left column
for (int r = bottom; r >= top; r--) res.add(matrix[r][left]);
left++;
}
return res;
}
}
``````